In mobile radio systems which do not transmit and receive continuously, but rather operate for example with time-division multiple access methods, it is necessary to regulate or to control the output power at an antenna of a mobile station in accordance with the respective specification. Before the beginning of a transmission time slot, it is necessary to regulate the transmission power upward within a predetermined time interval and to regulate it downward again after the end of the transmission time slot. This is referred to as power tamping and is necessary in order not to impermissibly disturb the signal transmissions in preceding or subsequent time slots, the so-called bursts.
A further requirement of the power regulation of a mobile station results from the mobile station's transmission power which is controlled by a base station and which is dependent on the respective received field strength. In order furthermore to prevent an impermissible disturbance of adjacent frequency channels, the transient spectrum during the upward and downward regulation of the transmission power must not have any abrupt changes nor any overshoots.
The output power of a power amplifier, which is usually arranged on the output side in a mobile radio transmitter and amplifies a radio frequency signal to be transmitted, depends on the ambient temperature, the battery or accumulator voltage, the respective transmission frequency, variations in manufacturing parameters during circuit integration, etc. Therefore, the output power of said power amplifier is usually stabilized by means of a regulating circuit.
Different power regulation principles are known depending on the modulation method used: in mobile radio systems which operate with phase or frequency modulation, the transmission power is usually carried out by variation of the supply voltage of the power amplifier by means of a field-effect transistor or by adjustment of the operating point of the amplifier; in mobile radio systems which operate with amplitude modulation, a controllable preamplifier is connected upstream of the power amplifier and regulates the input power of the power amplifier.
In one prior art example, the output power of the power amplifier is measured with the aid of a power detector, formed as a Schottky diode. This measured quantity is fed to an analog regulating circuit, which in turn drives the power amplifier. The reference quantity for said analog regulating circuit is supplied by a digital/analog converter driven by the baseband signal processing. The amplifier itself and also the control unit for said power amplifier are in this case arranged in different integrated circuits or constructed with discrete semiconductor components.
Another prior art example specifies a transmission arrangement with a direct modulator. The baseband signal processing unit is driven in a manner dependent on the power level of the transmission signal. A conventional automatic gain control is additionally provided between modulator output and antenna.
In another prior art example additional multipliers are provided in the baseband of a transmitter and condition the useful signal before modulation onto a carrier in a manner dependent on a power level received with other mobile units.
It is an object of the present invention to specify a transmission arrangement with power regulation which has a low current requirement.